1. Field of the Invention
The present invention relates generally to the field of telephone communications. More specifically, the present invention discloses a system for providing broadband audio program telephony circuits using 2B1Q technology.
2. Statement of the Problem
2B1Q line coding is an analog modulation technique for transmitting data over telephone lines. For example, this modulation technique has been selected to implement the U-interface in the ISDN standard. Using 2B1Q coding, binary data bits are grouped in pairs. Each pair is then transmitted as a quaternary symbol, the magnitude of which may be one out of four equally spaced voltage levels (-3, -1, +1, or +3). The quaternary symbols are transmitted at half the rate of the binary sequence. Therefore, an incoming binary stream with a bit rate of 160 kbit/sec. is converted into a quaternary stream with a symbol rate of 80 kbit/sec. An example of 2B1Q line coding is shown in FIG. 5. The 2B1Q modulation scheme is an important improvement over traditional analog transmission schemes which are significantly affected by circuit impairments that degrade normal analog transmission, such as impulse noise or cross-talk. The 2B1Q scheme utilizes integrated circuits that include transceiver circuits, echo cancellation, control circuitry, interface circuitry, and which provide for bi-directional transmission over a single pair of wires.
A number of systems using 2B1Q line coding have been invented in the past, including the following:
______________________________________ Inventor U.S. Pat. No. Issue Date ______________________________________ Wong, et al. 4,896,335 Jan. 23, 1990 Sayar 4,896,334 Jan. 23, 1990 Hulth 4,912,725 Mar. 27, 1990 Batruni, et al. 4,926,472 May 15, 1990 Aly, et al. 4,964,118 Oct. 16, 1990 ______________________________________
Wong discloses a digital 2B1Q transmitter that utilizes a dual modulator to generate a 2B1Q coded output by summing two binary modulated vectors in a 2:1 weighting ratio, allowing one modulating envelope generator to drive the dual modulator. The modulating envelope is coded in a 1-bit pulse density modulation (PDM) format, permitting the use of simple gating functions in performing the modulation function.
Sayar discloses a method for timing recovery in a receiver for a loop interface of a digital data transmission system, using for example 2B1Q coding.
Hulth discloses an adaptive equalizer for a receiver in a data transmission system using 2B1Q coding.
Batruni, et al., disclose a signal processing technique for removing echo interference or distortion from a received 2B1Q coded signal.
Aly, et al., disclose a system for echo cancellation in ISDN systems using 2B1Q coding.
3. Solution to the Problem
None of the prior art references uncovered in the search show a telephone system for broadband audio programming using 2B1Q technology between the remote location and the first central office, and between the second central office and the base station. The 2B1Q signal is translated into the appropriate digital format (e.g. T1) for transmission on the carrier between the central offices. The system uses multiple channels in each frame transmitted on the T1 carrier system to provide wide bandwidth.